As is well known, measurement data such as transmission characteristics analyzed or measured by a network analyzer, or an impedance which is analyzed or measured by an impedance analyzer, is given as a vector defined by an amplitude and a phase. When the measurement data expressed by such a vector is converted into polar coordinates and displayed as an image, whose amplitude and phase can be read from a single curve of a graph. In this case, it is convenient to use an image display unit employing a raster scan scheme. This is because a conventional image display unit (CRT display), represented by a television monitor, can be used.
FIG. 4 shows a conventional polar coordinate display device employing the raster scan scheme. For example, microprocessor 2 in signal analyzer 1, such as a network analyzer, converts vector data analyzed by signal analyzer 1 into polar coordinates, and sets the polar coordinate data and address data for image memory 4, which stores the polar coordinate data in graphic display controller 3 via data and address buses DB and AB. Graphic display controller 3 sets, in image memory 4, address data therefor set by microprocessor 2, and writes graph data (to be described later), based on the polar coordinate data converted by microprocessor 2, at a set address of image memory 4. Image memory 4 has a memory format having a one-to-one correspondence to the screen display format of image display unit 8 of a raster scan scheme such as a CRT. Graphic display controller 3 reads out data of image memory 4 by a timing pulse generated by timing circuit 7 in synchronism with raster scan by image display unit 8. In this case, a write-in cycle and a read-out (raster scan) cycle, in accordance with which graphic display controller 3 accesses image memory 4, are executed in a time divisional manner. Parallel-bit data read out from image memory 4 is converted into serial-bit data in parallel-serial converter 5 by a timing pulse generated by timing circuit 7. The serial-bit data is converted into a video signal via video circuit 6 and supplied to image display unit 8, so that the vector data analyzed by signal analyzer 1 is displayed as a graph curve of polar coordinate data.
When the vector data analyzed or measured by signal analyzer 1 is displayed in a real time manner to correspond to measurement scan, the graph of the coordinates which corresponds to the respective measurement point, i.e., previous graph data written in image memory 4, must be deleted, while displaying new data on the same coordinate system.
FIG. 5 shows an orthogonal coordinate system wherein a measurement scan direction and a graph display direction (to be referred to as a trace direction hereinafter) are the same, and the pitch of graph display (to be referred to as a trace hereinafter) is constant. In this case, no inconvenience occurs when the previous data is deleted and the new data is displayed in the case described above. This is because a graph of new point P in the measurement scan direction can be displayed by deleting data of the graph on point P from an image memory and writing new data in the image memory.
In contrast to this, FIG. 6 shows a polar coordinate system wherein a horizontal direction is expressed by X=Acos.theta. and a vertical direction is expressed as .gamma.=Asin.theta.. In this case, a measurement scan direction and a graph trace direction are generally different. In polar coordinate representation, the pitch of the graph trace is different from that of orthogonal coordinate representation, and is not constant. For this reason, when the vector data analyzed by signal analyzer 1 is displayed in accordance with the raster scan scheme as a polar coordinate on image display unit 8 by the conventional polar coordinate display device, part of the displayed graph becomes discrete, as shown in FIG. 7, so that the displayed graph is discontinuous. When the discontinuity is eliminated, the entire graph cannot be displayed in a real time manner.
This drawback will be described so that it can be understood easily. FIG. 8 is a partially enlarged view of FIG. 6. Assume that the trace of a graph is displayed by plotting points P10, P11, P12, . . . at a certain measurement scan. Points P10, P11, P12, . . . correspond to data of the respective measurement points analyzed by signal analyzer 1. When data corresponding to point P11 is written in image memory 4, graphic display controller 3 performs interpolation to smoothly connect points P10 and P11. The interpolated data, i.e., the graph data (including data corresponding to points P10 and P11) is also written in image memory 4. The same operation is performed for points P12, P13, . . . . Therefore, when data stored in image memory 4 is read out in a first measurement scan, a continuous graph is displayed on the screen of display unit 8, as shown in FIG. 6.
In a next measurement scan, assume that the positions of the respective measurement points of data analyzed by signal analyzer 1 are those represented by P20, P21, P22, . . . in FIGS. 6 and 8. In this case, when the graph data between points P20 and P21 is written in image memory 4, graphic display controller 3 first deletes the graph data connecting points P10 and P11 of the previous measurement scan, and writes the graph data between points P20 and P21 in image memory 4. Subsequently, when graph data between points P21 and P22 is written in image memory 4, graphic display controller 3 first deletes the graph data between points P11 and P12 of the previous measurement scan, and writes the graph data between points P21 and P22 in image memory 4. As a result, in the measurement scans following the second measurement scan, the graph data between points P11 and P21 is deleted, and the portion between points P11 and P21 is displayed as a discontinuous portion on the CRT screen. The portion between points P12 and P22 becomes discontinuous in the same manner. In this way, with a conventional circuit configuration, a displayed graph based on polar coordinate data is discontinuous, as shown in FIG. 7.
In order to prevent discontinuity in graph with the conventional circuit configuration, after a certain measurement scan is completed, all of the graph data of the measurement scan written in image memory 4 must first be deleted, and then new graph data of a next measurement scan must be written in image memory 4. In this case, however, every time measurement scan is performed, previous graph is deleted from the screen. As a result, such a conventional circuit configuration cannot be used in a measurement device which preferably displays vector data, analyzed by analyzer 1, as a polar coordinate by the raster scan scheme in a real time manner.